Deck Board Calculator
Estimate the number of deck boards, linear footage and material cost needed for your deck build.
This deck board calculator converts your deck's length and width, along with your chosen board width, board length and the gap you plan to leave between boards, into the number of deck boards you need to buy. Enter your dimensions, pick a waste percentage for cuts and irregular layouts, and optionally add a price per board to estimate total material cost.
Your Estimate
Total Deck Area
240.0 ft²
Deck Boards Required
52 boards
Boards Including Waste
58 boards
Estimated Linear Feet
624.0 ft
Linear Feet Including Waste
696.0 ft
Rows fill the deck width (12 ft); boards span the deck length (20 ft). A 0.125" gap is counted between rows only, not at the two outer edges. Enter the width of the board-covered field after allowing for required perimeter clearances. The last row may need trimming.
Results Actions
Estimates are approximate. Actual board counts can vary with decking pattern, framing layout, picture framing/borders, and cut waste at edges.
Common Deck Board Sizes
Actual board widths vary slightly by mill and manufacturer — measure a sample board before finalizing your order.
Common Board Gaps
How to Use the Deck Board Calculator
- 1Measure the length and width of your deck in feet (or meters).
- 2Choose your deck board width (5/4×6, 2×6, 2×8, or enter a custom width).
- 3Choose the board length you plan to buy (8 ft, 10 ft, 12 ft, 16 ft, 20 ft, or a custom length).
- 4Set the gap you will leave between boards — 1/8 in is standard for wood, check the spec for composite.
- 5Choose a waste percentage — 10% for simple layouts, 15%+ for diagonal patterns or borders.
- 6Optionally enter a price per board to see an estimated material cost.
Example Calculation
Say you're building a 20 ft × 12 ft deck using 12 ft long 2×6 boards (5.5 in actual width) with a 1/8 in gap and a 10% waste factor. Deck area = 20 × 12 = 240 sq ft. Rows needed to cover the 12 ft field width = ceiling((144 + 0.125) ÷ (5.5 + 0.125)) = 26 rows, with gaps only between rows. Since each row of 12 ft boards can only span 12 ft of the 20 ft deck length in one piece, you need 2 boards per row (26 × 2 = 52 boards) before waste. With 10% waste, boards including waste = 58 boards, or about 696 linear feet.
Accuracy & Assumptions
- Rows include gaps only between boards, with no outside edge gaps. The final row may need trimming.
- Board coverage and lengths can vary by manufacturer and material (wood vs. composite).
- The selected waste percentage is already included in the purchase count; avoid adding it twice.
- Verify deck dimensions and board spacing before purchasing materials.
- Check manufacturer span ratings and local building codes for joist spacing and fasteners.
Frequently Asked Questions
How many deck boards do I need?
Convert the board-covered field width to inches or millimeters, add one gap, divide by actual board width plus gap, and round up. This counts gaps only between rows. A 12 ft field with 5.5 in boards and a 1/8 in gap needs 26 rows. Multiply rows by the whole stock pieces needed to span the deck length.
What gap should I leave between deck boards?
Leave about 1/8 inch (3 mm) between pressure-treated or cedar boards to allow for drainage and seasonal wood movement. Composite boards often need a wider gap — check the manufacturer specification, which is typically around 3/16 inch.
Should I run deck boards the long way or across the joists?
Deck boards are almost always installed perpendicular to the joists for proper support. This calculator places rows across Deck width and runs boards along Deck length. Swap the entered dimensions deliberately to model the other direction; confirm the actual framing layout separately.
How much waste should I add for a deck board order?
Use 10% waste for a simple rectangular deck, and 15% or more if your design includes diagonal patterns, picture-frame borders, curves, or multiple angle cuts.
Does this calculator work for composite decking?
Yes. Choose the custom board width option to enter the actual width of your composite board and adjust the gap to match the manufacturer's recommended spacing (commonly 3/16 inch) before calculating.
This calculator provides an estimate for planning purposes only. Actual board counts, sizes and pricing vary by supplier and material — always confirm final quantities with your lumber yard or decking supplier before purchasing, and consult a licensed contractor for structural framing and joist spacing.